Source: Auckland Council
What causes a sinkhole or a tomo?
Sinkholes, also known as ‘tomo’, form when soil under the ground surface is eroded by water. This can happen naturally or as a result of human activities.
In some cases, they occur naturally due to underground springs or streams. For example, lava tunnels can collapse, or limestone landscapes, like those in Waitomo, can develop sinkholes over time.
Heavy rain increases the risk, especially when the ground is already saturated.
Sinkholes can also be caused by human activities like underground water pipes leaking, including damage to wastewater and stormwater systems.
Sinkholes are not uncommon, especially in areas with ongoing rain, silty soil, or high soil saturation — conditions that Auckland experiences regularly.
What is the difference between a sinkhole and a pothole?
Potholes, while they do not pose as much of a safety risk as sinkholes, can cause damage to vehicles.
Potholes form from the surface when water seeps into small cracks in the road. When temperatures change, the water expands and contracts, breaking down the road material. Traffic makes the problem worse, as the weight of vehicles causes the weakened surface to crack and crumble. Over time, this leads to a shallow hole forming at the surface of the road. They are usually limited to the thickness of the surface layers of the road.
Sinkholes are quite different. They start to form from a small hole underneath the ground that eventually grows large enough to reach the surface. They will typically be much deeper and therefore more dangerous than a pothole.
Is there any way of knowing when a sinkhole may appear?
We can’t predict accurately when and where a sinkhole may appear. This is because of the huge size of the utility network, the types of soil we have in Auckland, and the unpredictable nature of erosion.
Auckland has a vast number of utilities including stormwater and wastewater network across the city located across the city, making it challenging to quickly identify all leaks, especially when these are underground and not visible at the surface.
A lot of our soils are silty, which is a size of material that is particularly easy for slow-flowing water to erode. Water from leaking infrastructure often flows a significant distance from the source of any leak before it starts to erode enough material to form a sinkhole, making it hard to identify whether the source of the water is due to a leak or due to natural groundwater. The erosion can happen slowly over a long period of time before a small change causes a collapse.
Do you have any way of predicting of how many there might be?
We can’t predict how many there might be. We can infer numbers from looking at the numbers we have had in previous years, but we don’t have a predictive model that can accurately predict how many will occur.
What is Auckland Council doing to lower the risk of sinkholes appearing?
Auckland Council looks after stormwater, while Watercare looks after wastewater and water supply.
Approximately 7,000 km of stormwater pipeline in Auckland is maintained by the council’s Healthy Waters and Flood Resilience team.
There is a proactive programme in place to upgrade and replace sections of this pipeline based on criticality, with factors such as pipe age, material, location and service type affecting prioritisation.
There’s also the Making Space for Water programme which includes a range of initiatives to reduce flood risk to Aucklanders. Part of this is building new flood resilient infrastructure to enhance stormwater assets and green spaces to deliver increased flood management as well as increasing the inspections of our pipes.
What should members of the public do if they see a sinkhole?
Most importantly, from a safety perspective, don’t drive around or walk up to a sinkhole. Be very careful and avoid approaching a sinkhole, it may be a bigger underground than the hole appears on the surface.
Please contact Auckland Council on 09 301 0101 with the address and we will attend to it as quickly as possible. Our customer experience team can then triage and ensure we send our contractors our immediately, to ensure the safety of the public.
It’s important to remember that Auckland Council is a large and complex organisation. The issue could sit with stormwater infrastructure (managed by Auckland Council), wastewater infrastructure (managed by Watercare) or the road itself (managed by Auckland Transport).
